Distributed feedback ring laser

ABSTRACT

A single longitudinal mode narrow linewidth and low threshold all fiber laser arrangement is disclosed, constructed from a ring cavity, a gain portion in the ring cavity, an input-output coupling interposed within the cavity and pumping means connected to the ring cavity and to the grating such that, upon actuation of the pumping means, the arrangement acts as a laser.

FIELD OF THE INVENTION

The present invention relates to a single frequency laser arrangementand, in particular, to a single longitudinal mode narrow linewidth andlow threshold all fibre laser arrangement.

BACKGROUND OF THE INVENTION

Single frequency lasers are particularly useful in optical communicationsystems, fibre sensors, and spectroscopy as narrow linewidth sources.There has been increasing interest in developing single longitudinalmode fibre lasers.

One approach to obtain single longitudinal mode operation is to use adistributed feedback structure (DFB). As an example, reference is madeto H Kogelnik and C V Shank, “Coupled-wave theory of distributedfeedback lasers”, Journal of Applied Physics vol. 43, no. 5, pp.2327-2335, 1972. Recently, DFB fibre lasers have been demonstrated,using in fibre gratings directly written into rare earth doped fibres[J. T. Kringlebotn, J-L Archambault, L. Reekie, and D. N. Payne,“Er³⁺:Yb³⁺-codoped fibre distributed-feedback laser”, Optics Letters.,vol. 19, no. 24, pp. 2101-2103, 1994 and A. Asseh, H. Storoy, J. T.Kringlebotn, W. Margulis, B. Sahlgren, S. Sandgren, R. Stubbe, and G.Edwall, “10 cm Yb³⁺ DFB fibre laser with permanent phase shiftedgrating”, Electron. Letters., vol. 31, no. 12, pp. 969-970, 1995]. Usingfibre DFB structures permits all fibre devices and wavelengthselectivity over a wide range. However, the use of short pieces ofheavily doped fibre has a disadvantage of low slope efficiency and largelinewidth. Another approach to obtain single longitudinal mode operationis to use the travelling wave operation of ring fibre lasers [G. J.Cowle, D. N. Payne, and D. Reid, “Single-frequency travelling-waveerbium-doped fibre loop laser”, Electron. Letters., vol. 27, no. 3, pp.229-230, 1991]. Long cavity lengths of ring lasers provide narrowlinewidth operation. However, single longitudinal mode operation of ringlasers reported to date has been achieved mostly with pigtailednon-fibre intracavity elements.

Suppression of spatial hole burning by internal modulation of the lasercavity [T. Stolte and R. Ulrich, “Er-fibre lasers: suppression ofspatial hole burning by internal modulation”, Electron. Lett., vol. 29,no. 19, pp. 1686-1688, 1993] can be used to ensure single longitudinalmode operation. However, it also requires pigtailed non-fibreintracavity elements.

Further, single longitudinal mode narrow linewidth operation can beachieved using injection locking [J. D. C. Jones and P. Urquhart, “Aninjection-locked erbium fibre laser”, Optical Communications., vol. 76no. 1, pp. 42-46, 1990]. However, this approach requires an externalsingle longitudinal mode narrow linewidth laser.

The disadvantage of using a coupled cavity fibre laser incorporatingfibre Bragg gratings [S. V. Chernikov, J. R. Taylor, and R. Kashyap,“Coupled-cavity erbium fibre lasers incorporating fibre gratingreflectors”, Opt. Letters., vol. 18, no. 23, pp. 2023-2025, 1993] orFox-Smith fibre lasers [P. Barnsley, P. Urquhart, C. Millar, and M.Brierley, “Fiber Fox-Smith resonators: application tosingle-longitudinal-mode operation of fibre lasers”, Journal of theOptical Society of America vol. 5, no. 8, pp. 1339-1346, 1988] is thatthese approaches require several perfectly matched gratings, althoughthe approaches permit all fibre devices.

A saturable absorber [M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind,“Narrow-linewidth, singlemode erbium-doped fibre laser with intracavitywave mixing in saturable absorber”, Electron. Letters., vol. 30, no. 8,pp. 648-649, 1994] can be used to achieve single longitudinal modenarrow linewidth operation. However, the absorber increases the lasingthreshold and reduces slope efficiency of the laser.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved laserarrangement which overcomes some of the disadvantages of theaforementioned laser arrangements.

In accordance with the first aspect of the present invention there isprovided a laser arrangement which comprises a ring cavity with a gainportion, an input-output coupling connected to the ring cavity, agrating interposed in the gain portion of the ring cavity, and a pumpingmeans connected to the gain portion of the ring cavity and the grating,such that, upon activation of the pumping means, the laser arrangementoperates as a laser and produces an output at said input-outputcoupling.

Preferably, the ring cavity and grating are formed from the one piece ofrare earth doped optical fibre with the grating being written directlyinto the optical fibre.

Preferably, the laser arrangement acts as a distributed feedbackstructure for obtaining a single longitudinal mode of operation over afrequency linewidth of less than 70 kHz.

In broad terms, the preferred embodiment of the present inventionprovides a single longitudinal mode narrow linewidth and low thresholdlaser, comprising a distributed feedback structure inserted into a ringcavity.

More specifically, the preferred embodiment of the present inventionprovides a single longitudinal mode narrow linewidth and low thresholdlaser arrangement, comprising a distributed feedback structure forobtaining single longitudinal mode operation, and an output coupler toprovide output coupling and additional feedback through the ring cavity.The additional feedback of the lasing signal via the ring cavitydecreases the lasing threshold because of the reduction in effectiveresonator losses. The additional feedback also leads to laser linewidthnarrowing provided by self injection of the lasing signal through thering cavity which can be considered as an extended cavity.

In a preferred embodiment the distributed feedback structure is agrating written into an Er³⁺:Yb³⁺-doped phosphosilicate fibre with atransmission peak within the corresponding photonic bandgap. Splicing ofthis structure within a ring cavity has resulted in a singlelongitudinal mode operation at the transmission peak wavelength. Such anarrangement has advantages in producing single longitudinal mode narrowlinewidth and low threshold laser sources in optical communicationsystems, fibre sensors, and spectroscopy.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become apparentfrom the following description of embodiments thereof, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a laser arrangement in accordance withan embodiment of the present invention;

FIG. 2 shows the calculated single pass gain threshold of the laser vsoutput coupling for grating strength κL=2π/3 and uniform gain in a DFBstructure;

FIG. 3 is a DFB transmission spectrum with (solid line) and without(dashed line) pumping, wherein the DFB structure employed is a gratingwritten into an Er³⁺/Yb³⁺-doped phosphosilicate fibre;

FIG. 4 is an optical spectrum of the laser arrangement of FIG. 1,measured by a monochromator (0.1 nm resolution);

FIG. 5 shows an output spectrum of the laser arrangement of FIG. 1,measured by scanning Fabry-Perot interferometer 5 MHz resolution); and

FIG. 6 is spectral linewidth of the laser arrangement of FIG. 1,measured by delayed self-heterodyne technique (5 kHz resolution).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates a ring DFB laser arrangement 1comprising a DFB structure 2 coupled to itself via a ring cavity 5. Thearrangement also comprises an output coupler 3 to provide outputcoupling and to complete the ring. A wavelength division multiplexing(WDM) fibre coupler 4 is coupled to and provides optical pumping of theDFB structure 2.

The ring cavity 5 provides additional feedback to the lasing signal ofthe DFB structure 2. This feedback decreases the lasing thresholdcompared to the traditional DFB laser because of the reduction in theDFB structure losses to output. To determine the efficiency arising fromthe feedback, the predicted lasing threshold of the λ/4-shifted DFBstructure within the ring as a function of coupling ratio of the outputcoupler is plotted in FIG. 2 for a grating strength κL=2π/3, assuminguniform gain along the grating. It can be seen that a considerablereduction in the threshold can be achieved with only half of the DFBstructure output re-injected via the ring 5. The pump threshold, whichdepends on a doped fibre and pump employed, can be improved further byoptimizing the output coupler 3 at the pump wavelength.

In a first example, Er³⁺/Yb³⁺-doped phosphosilicate fibre (1 wt. %Er₂O₃, 10 wt. % Yb₂O₃, 33 wt. % P₂O₅) was produced by the flashcondensation technique [A. L. G. Carter, M. G. Sceats, S. B. Poole,“Flash condensation technique for the fabrication of highphosphorous-content rare-earth-doped fibres”, Electron. Letters., vol.28, no. 21 pp. 2009-2011, 1992], and was used to form the arrangement 1.A saturated gain measurement at 1535 nm was about 0.8 dB/cm whenmeasured with a 980 nm pump laser diode. A 3 cm long grating 2 withreflection R>99% was fabricated by exposing a hydrogenated sample (100atm, 373K) to the 193 nm output of an ArF excimer laser through a 1535nm phase mask. The grating 2 was then pumped with a 980 nm pump laserdiode through a 980/1550 WDM 4 located in the ring.

FIG. 3 shows a graph of the resulting transmission spectrum of thegrating 2 with pumping 10 and without pumping 11, using a narrowlinewidth tunable semiconductor laser source with a 0.001 nm settingaccuracy. The coupling coefficient was estimated from the transmissionspectrum of the grating to be κL≈3. As can be seen from FIG. 3, whenpumping the grating 2, the centre wavelength generally shifts to longerwavelengths with the simultaneous appearance of the transmission peak 12within the photonic bandgap at a certain pump intensity. In the presentcase, splicing of the DFB structure 2 within the ring with 1% outputcoupling led to lasing at the transmission peak wavelength, the opticalspectrum of which is shown in FIG. 4, when utilising a threshold pumppower of approximately 10 mW. It should be noted from FIG. 4 that sidemode suppression was more than 50 dB.

Lasing was found to be robustly in a single longitudinal mode, confirmedusing a scanning Fabry-Perot interferometer. FIG. 5 shows thelongitudinal mode spectrum of the laser output, while FIG. 6 shows thelinewidth of the ring DFB laser that was measured to be 6.5 kHz using aconventional self heterodyne technique with a 20 km delay line.

The above arrangement 1 has a potential tunability over a widewavelength range by tuning the centre wavelength of the grating and/ortransmission peak within the photonic bandgap of the grating.

It will be appreciated that the present invention is not limited to anall fibre arrangement and that other distributed feedback structures andring cavities may be employed.

It will be further appreciated that the present invention is not limitedto a λ/4 shifted distributed feedback structure and that otherdistributed feedback structures both gain and/or index modulated, and/orwith complex modulation profiles, such as sampled Bragg gratingstructures or chirped Bragg grating structures, may be employed.

When complex distributed feedback structures are employed, it can beappreciated that the laser configuration is able to generate dual ormultiple laser modes each of which will have low-threshold, stable andnarrow-linewidth operation similar to that of the single-mode ring DFBlaser.

It will be further appreciated that other dopants apart from erbium maybe used in the distributed feedback structure.

It will be appreciated by persons skilled in the art that furthervariations and/or modifications may be made to the invention as shown inthe specific embodiment without departing from the spirit or scope ofthe invention as broadly described. The present embodiment is,therefore, to be considered in all respects as illustrative and notrestrictive.

What is claimed is:
 1. A distributed feedback ring laser arrangementcomprising: a ring cavity; a gain portion in said ring cavity; an inputcoupling connected to said ring cavity; an output coupling connected tosaid ring cavity; a grating interposed within the gain portion of saidring cavity; and an optical pumping means connected to said gratingwithin said gain portion of said ring cavity such that upon activationof said optical pumping means, said laser arrangement operates as adistributed feedback ring laser producing a laser output at said outputcoupling.
 2. A laser arrangement as claimed in claim 1 wherein said ringcavity comprises an optical fibre.
 3. A laser arrangement as claimed inclaim 2 wherein said optical fibre has a rare earth doped core.
 4. Alaser arrangement as claimed in claim 3 wherein said grating comprises aBragg grating directly written into said optical fibre.
 5. A laserarrangement as claimed in claim 3 wherein said rare earth is erbium. 6.A laser arrangement as claimed in claim 1 wherein said grating isarranged to obtain single longitudinal mode operation of said laser. 7.A laser arrangement as claimed in claim 1 wherein said ring cavityprovides a reduced threshold and provides linewidth narrowing of saidlaser.
 8. A laser arrangement as claimed in claim 7 wherein thelinewidth of said laser is less than 70 kHz.
 9. A laser arrangement asclaimed in claim 1 wherein said laser is tuned to a predeterminedfrequency by tuning of said grating.
 10. A laser arrangement as claimedin claim 1 wherein said ring cavity has a λ/4 shifted distributedfeedback structure.
 11. A laser arrangement as claimed in claim 1wherein said grating is a chirped Bragg grating.
 12. A laser arrangementas claimed in claim 1 wherein said grating is a sampled Bragg grating.13. A distributed feedback ring laser arrangement comprising: a ringcavity; a gain portion in said ring cavity; an input coupling connectedto said ring cavity; an output coupling connected to said ring cavity; agrating interposed within the gain portion of said ring cavity; and apumping means connected to said grating within said gain portion of saidring cavity such that upon activation of said pumping means, said laserarrangement operates as a distributed feedback ring laser producing alaser output at said output coupling; wherein said ring cavity and saidgrating comprise an optical fibre.
 14. A laser arrangement as claimed inclaim 13 wherein said optical fibre has a rare earth doped core.
 15. Alaser arrangement as claimed in claim 13 wherein said grating comprisesa Bragg grating directly written into said optical fibre.
 16. A laserarrangement as claimed in claim 14 wherein said rare earth is erbium.17. A laser arrangement as claimed in claim 13 wherein said grating isarranged to obtain single longitudinal mode operation of said laser. 18.A laser arrangement as claimed in claim 13 wherein said ring cavityprovides a reduced threshold and provides linewidth narrowing of saidlaser.
 19. A laser arrangement as claimed in claim 18 wherein thelinewidth of said laser is less than 70 kHz.
 20. A laser arrangement asclaimed in claim 13 wherein said laser is tuned to a predeterminedfrequency by tuning of said grating.
 21. A laser arrangement as claimedin claim 13 wherein said ring cavity has a λ/4 shifted distributedfeedback structure.
 22. A laser arrangement as claimed in claim 13wherein said grating is a chirped Bragg grating.
 23. A laser arrangementas claimed in claim 13 wherein said grating is a sampled Bragg grating.